Retron Diels-Alder transform

Given structure 1 as a target and the recognition that it contains the retron for the Diels-Alder transform, the application of that transform to 1 to generate synthetic precursor 2 is straightforward. The problem of synthesis of 1 is then reduced retrosynthetically to the simpler... 

Once a particular 6-membered ring is selected as a site for applying the Diels-Alder transform, six possible [4 + 2] disconnections can be examined, i.e. there are six possible locations of the Jt-bond of the basic Diels-Alder retron. With ring numbering as shown in 36, and... 

Estrone (54, Chart 6) contains a full retron for the o-quinonemethide-Diels-Alder transform which can be directly applied to give 55. This situation, in which the Diels-Alder transform is used early in the retrosynthetic analysis, contrasts with the case of ibogamine (above), or, for example, gibberellic acid (section 6.4), and a Diels-Alder pathway is relatively easy to find and to evaluate. As indicated in Chart 6, retrosynthetic conversion of estrone to 55 produces an intermediate which is subject to further rapid simplification. This general synthetic approach has successfully been applied to estrone and various analogs. ... 

Structural subgoals may be useful in the application of transform-based strategies. This is especially so with structurally complex retrons which can be mapped onto a target in only one or two ways. It is often possible in such cases quickly to derive the structure of a possible intermediate in a trial retrosynthetic sequence. For instance, with 109 as TGT the quinone-Diels-Alder transform is an obvious T-goal. The retron for that transform can readily be mapped... 

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